Solenoid actuators are in widespread use in many applications, such as for actuating valves. A conventional, single-acting solenoid actuator typically has a pole member, a wire coil provided in the pole member, a linearly translatable armature associated with the pole member, and a spring for biasing the armature away from the pole member. When electric current is provided to the wire coil, a magnetic field is produced which overcomes the force of the bias spring to draw the armature towards the pole member. When the electric current is switched off, the magnetic force dissipates, and the bias spring urges the armature away from the pole member. When the armature of the solenoid actuator is connected to the stem of a valve element, the operation of the solenoid controls whether the valve is opened or closed.
A non-latching type solenoid is one that requires an electric current to be provided to the wire coil in order to maintain the armature in a position adjacent the pole member, referred to herein as the "actuated position." In applications which require the armature to be in the actuated position for long periods of time, a non-latching solenoid actuator requires a relatively large amount of energy to operate and is therefore inefficient.
To overcome the above problem of high energy consumption, a conventional latching solenoid actuator may be used. Such a solenoid actuator does not require electric current to be provided to the wire coil at all times in order to maintain the armature in the actuated position.
One conventional type of latching solenoid actuator relies upon the magnetic attraction between the pole member and the armature caused by residual magnetism in those two elements after the electric current to the wire coil is switched off. In that type of actuator, to move the armature from its non-actuated position to its actuated position, the electric current is turned on until the armature makes contact with the pole member, at which point it is turned off, and the armature remains in its actuated position due to residual magnetism, which applies a holding force greater than the opposing force of the bias spring. To move the armature to its non-actuated position, electric current is temporarily provided in the opposite direction in the wire coil to cancel or oppose the residual magnetism, which allows the bias spring to move the armature to its non-actuated position. To achieve the necessary residual magnetism for operation, the pole member and the actuator are typically composed of soft magnetic materials, such as pure iron and/or 3% silicon iron.
Although a latching actuator of the type described above is advantageous in that it conserves electrical power, it is not suitable for applications which require numerous actuations of the solenoid actuator because the magnetic materials from which the pole member and armature are composed are relatively soft. As a result, the repeated contact between the pole member and armature results in mechanical wear on those two components, causing the amount of linear displacement of the solenoid actuator to gradually change over time. Such actuators are not acceptable for applications, such as fuel injection systems, requiring precise linear movements over an extended period of time. The mechanical wear of the armature and pole member may also generate small metallic particles which would contaminate the solenoid actuator and hinder its operation.
The wear problem described above usually cannot be overcome by providing a mechanical stop to prevent the armature from making contact with the pole member, resulting in a small air gap between the pole member and the armature when the armature is in the actuated position. This approach is not acceptable since the provision of the air gap usually weakens the residual magnetism enough so that it is insufficient to hold the armature in its actuated position after the electric current in the wire coil is turned off.
One manner in which the wear problem may be overcome is by providing an air gap between the pole member and the armature when the armature is in the actuated position and, instead of using residual magnetism to latch the armature, incorporating one or more permanent magnets in the pole member to hold the armature in its actuated position. However, the use of permanent magnets in solenoid actuators has drawbacks because permanent magnet materials are relatively expensive and because their magnetic characteristics change with temperature, thus rendering them unsuitable for some applications in which substantial temperature changes occur.
U.S. Pat. No. 3,743,898 to Sturman discloses various embodiments of a latching solenoid actuator which utilize residual magnetism to hold the actuator in the actuated position. Sturman indicates that the magnetic components of the actuator may be formed of various materials, such as C1010 and C1020 low carbon steel.
U.S. Pat. No. 4,114,648 to Nakajima, et al. discloses a double-acting electromagnetic valve in which a movable magnetic member 17 within the magnetic cores 1, 2 of the valve is latched in the two actuated positions by residual magnetism. Nakajima, et al. indicate that the magnetic cores 1, 2 may be composed of a magnetic material having a high residual magnetism such as magnet steel or heat-tempered, high carbon steel, such as S50C carbon steel after being subjected to a heat tempering process. The movable magnetic member 17 may be made of a magnetic material having a low residual magnetism or a high residual magnetism.
U.S. Pat. No. 4,231,525 to Palma discloses an electromagnetic fuel injector with a selectively hardened armature. The fuel injector includes a valve that may be made of any suitable hard material, either a magnetic material or a non-magnetic material. For durability, the valve may be made of suitably hardened SAE 51440 stainless steel. The armature is made of a magnetically soft material, such as SAE 1002-1010 steel. To prevent the armature from wearing during extended usage, Palma indicates that selected surfaces of the armature should be case hardened. In particular, the armature surfaces which should be hardened are those surfaces that are not within the magnetic circuit, but which are subject to wear during extended usage. The manner in which the armature surfaces are selectively case hardened is relatively complicated.